Method and device for producing hollow bodies

ABSTRACT

A method and device for producing hollow bodies comprising a gas barrier coating, in particular PET containers comprising a coating of polyvinyl alcohol. The hollow bodies are treated in a device for increasing the surface energy of the surface to be coated and then in a coating device and a drier. To improve the gas barrier effect of the gas barrier coating, a multi-stage pre-treatment is disclosed, in which the hollow body is subjected to an electrostatic discharge after the treatment for increasing the surface energy.

DESCRIPTION

The invention relates to a method and a device for the manufacture ofhollow bodies of the type described in the preambles of claims 1 and 8.

Large numbers of methods and devices for the manufacture of hollowbodies, in particular single-use and multiple-use containers made of apolyethylene terephthalate (PET), are known. In particular, drinkbottles made of PET are becoming increasingly predominant in the market,because they are very light, suitable for food, and they can bemanufactured in a cost advantageous manner and present sufficientmechanical resistance. However, one drawback of PET is the fact that itis permeable to gases, such as, for example, oxygen or carbon dioxide.If oxygen penetrates from the outside to the inside, or if carbondioxide escapes from carbon dioxide containing drinks from the inside tothe outside through the PET wall, this can lead, in both cases, to alarge decrease in the service life. For that reason, attempts are madeto lower the gas permeability using appropriate coatings. One method forthe manufacture of hollow bodies of the generic type is described, forexample, in DE-OS 29 05 480. In the known method, a preform made of thecontainer material is first coated with polyvinyl alcohol. Althoughpolyvinyl alcohol produces a very good gas barrier and is compatiblewith food, it is water soluble, and therefore the polyvinyl alcoholcoating, after it has dried, is provided with an additional coatingwhich is less sensitive to humidity. After this layer has also beendried, the preform is then shaped to the finished container, usually byblow molding, during which process the coatings are also shaped and thusthey cover the entire finished container. One problem here consists ofthe adhesion of the polyvinyl coating to the hollow body. In knownmethods, a surface treatment is used immediately before the firstcoating process to increase the surface energy, whereby the surfaceenergy is increased from approximately 38 mN/m for the untreated surfaceto a maximum of 52 mN/m. This process can be, for example, a coronatreatment or a flaming, or it can consist of the addition of asurfactant or similar agent to the polyvinyl alcohol. Nevertheless, itshould be noted that the barrier properties of the coating cannot beimproved as much as one would expect.

This problem is addressed in DE 27 17 307 D2. In that patent, it isnoted that bubbles form in a coating, which are thought to be caused bycarbon dioxide diffusing through the PET wall. In the method describedin this patent, an attempt is made to prevent this effect by applyingthe coating to the internal side of the hollow body. However, thecoating of the internal side of a hollow body is more complicated thanthe coating of the external side, and it is only suitable for a limitednumber of coating agents and not for all application purposes.

The invention is thus based on the problem of providing a method and adevice for the manufacture of hollow bodies with an improved gas barriercoating.

This problem is solved by the method according to claim 1 and the deviceaccording to claim 8.

The present invention is based on the knowledge that, using the claimed,careful, multi-step preliminary treatment, it is possible to effectivelysuppress bubble formation and the associated decrease in the barriereffect. As a result of the electrostatic discharging of the surfaceafter the treatment to increase the surface energy, the adhesion of thecoating can be evened out, so that bubble formation no longer occurs.

It is advantageous to carry out this discharge using an air shower withionized air, where, as a result of the mechanical blowing, any adhesivemicroparticles attracted by the electrostatic charge are also blown off.

If one expects, for example, fatty deposits originating from moldremoval agents in the case of injection molded preforms, then it isadvantageous and usually also necessary to remove the fat prior to atreatment to increase the surface energy.

An additional possibility to improve the barrier properties of thecoating consists in conferring a substantially higher surface energythan has been done to date in the state of the art. According to theinvention, the surface energy is increased to values above 60 mN/m, andpreferably above 70 mN/m. To increase the surface energy to thesevalues, it is appropriate above all to use the known flaming method.

An additional possibility to improve the barrier properties of thecoating, according to the invention, consists of a gentle blowingprocess at lower temperatures (below 60° C., preferably at 45° C.),using, however, a highly dehumidified air, which preferably has a watercontent of less than 3 g/m³. As a result of this type of drying, an evenand complete drying throughout the entire layer thickness is achieved,so that any remaining residual humidity content is distributed evenlythroughout the entire coating and the formation of a skin is suppressed.On the other hand, dryers, when used under microwave or infraredradiation, produce a more rapid drying, but they are associated with therisk that a higher than desired residual humidity content remains undera very dry, crumpled, surface skin, where the crumpled surface persistseven after the blowing process.

An additional possibility to improve the gas barrier properties consistsin applying the gas barrier coating by blowing, where one must ensurethat excess coating agent is applied to the surface to be coated in sucha manner that the coating agent is not immediately rinsed off again. Ifthe next step consists of a film formation section, then the excess candrip off slowly, so that a completely covering film has already formedbefore a stronger dryer effect is applied.

Embodiment examples of the invention are explained in greater detailbelow, with reference to the drawing. In the drawing:

FIG. 1 shows a schematic top view of a device according to the inventionfor carrying out the method according to the invention, and FIG. 2 showsa schematic representation of a hollow body during the coating.

FIG. 1 shows a highly schematic top view of a part of a substantiallyautomatic and continuous device 1 for the manufacture of hollow bodies 2(FIG. 2) with a gas barrier coating 3. The hollow bodies 2 can be theconventional preforms (preforms), as used for manufacturing drinkbottles or similar containers. However, the device 1 according to theinvention can also be used to coat hollow bodies which have already beenfinished.

The hollow bodies 2, which are designed as preforms, consists, in thepreferred embodiment example, of a polyethylene terephthalate (PET)which is conventionally used for manufacturing drink bottles. Suchpreforms present an already finished threaded head 2 a, which isseparated by a projecting flange or collar 2 b from the body 2 c proper,which is to be coated and has closed ends. The projecting flange 2 b, onthe one hand, serves as a holding element for known transport orgripping devices, by means of which the hollow bodies 2 can betransported individually suspended and optionally with rotation in thedirection of the arrow A about their longitudinal axis through thedevice 1. On the other hand, the flange protects the already finishedthreading during the processing.

Using, for example, a conveyor 4, the hollow bodies 2 are transferredindividually one after the other into the device 1, which can beoptionally encapsulated, so that operations under clean-room conditionsare possible. However, it is also possible to connect the device 1directly to a manufacturing section for the hollow bodies 2.

From the conveyor 4, the hollow bodies 2 are transferred to a linearconveyor 5, which extends through the entire device 1 and which can be,for example, in the form of an endless conveyor chain fitted with spikeswhich can be inserted into the preforms. This conveyor 5 first conveysthe hollow bodies 2 through a multi-step preliminary treatment section6. The preliminary treatment section 6 contains, in the conveyancedirection F, a device 7 for degreasing after the conveyor 4. The device7 preferably contains washing nozzles, which should be directed onto thepart 2 c to be coated of the hollow body 2. It is preferred for theconveyor 5 to be designed in this area in such a manner that the hollowbodies 2 rotate. It is preferred to use ethyl alcohol (ethanol) for agreasing agent, and to spray it through nozzles onto the hollow body, sothat the latter is completely wetted.

In the conveyance direction F which follows, the preliminary treatmentsection 6 contains a device 8 to increase the surface energy. It ispreferred for the device 8 to be a flaming device, as already used forthis purpose in the state of the art. However, the flaming device 8 isadjusted in such a manner that the surface energy can be increased tovalues above 60 mN/m and preferably above 70 mN/m. In the area of thedevice 8, the conveyor device 5 is also designed in such a manner thatthe hollow body 2 rotates about a vertical axis.

The preliminary treatment section 6 contains, in the conveyancedirection F after the flaming device 8, a device 9 to lower theelectrostatic charging or to discharge the surface to be coated of thehollow body 2. The device 9 in turn contains nozzles which are directedonto the area 2 c to be coated of the body 2, through which jets ofionized air are blown against the hollow body 2. As a result of theionized air, the surface is discharged, and at the same time anyadhering particles are blown off by the air jets. In the area of thedevice 9, the conveyor device 5 is also designed in such a manner thatit rotates the hollow bodies 2 about their vertical axis.

After completion of the preliminary treatment section 6, the coatingproper occurs, preferably on the external side of the hollow body whichis still warm from the flaming. In the process, the hollow body 2 passesthrough a first coating device 10, in which a gas barrier layer 3,preferably based on polyvinyl alcohol, is applied. The coating agentsfor such gas barrier layers are known to the person skilled in the artand they do not have to be explained further here. It is preferred touse a polyvinyl alcohol with a high hydrolysis degree of more than 99.9mol %, which is dissolved in fully desalted water. The coating agentpreferably contains no additional additives, in particular no wettingagent. The coating agent has a concentration of 5-12% and it isprocessed at room temperature.

The coating agent is preferably applied by blowing, as shown in FIG. 2,where a nozzle 11 leads an aqueous solution of polyvinyl alcohol 12 in ablow jet against the surface 2 c to be coated, where the nozzle M andhollow body 2 move relative to each other; and it is preferred for thehollow body 2 to rotate. The quantity of the coating agent 12 exceedsthe quantity which would be sufficient for to completely cover thesurface 2 c to be coated, so that the excess drips off in drops 12 a.The upper edge of the nozzle opening 11 a of the nozzle 11 is located ata separation distance a below the collar 2 b, where the separation a ismeasured in such a manner that the coating agent 12 is blown to alocation below the collar 2 b without going over the collar 2 b, sothat, in this manner, the threading 2 a is effectively protected againstundesired coating. The separation a, in the case of the hollow bodies 2which are usually used for the manufacture of PE bottles, isapproximately 20-25 mm, but it depends on the special viscosity of thecoating agent 12 and on the quantity used.

The viscosity and the quantity of the coating agent 12 is chosen in sucha manner that, on the surface 2 c to be coated, a single layer withsufficient layer thickness of the coating agent 12 continues adhering,where this single layer still guarantees a complete coverage with athickness of approximately 2 μm, even after the blowing of the hollowbodies 2 to form the finished containers.

After the first coating device 10, a drip and film formation section 13follows, in which the final layer thickness is established.

Then the conveyor 5 leads the hollow bodies 2 into a first dryer 14. Thefirst dryer 14 presents a closed housing 14a, in whose interior, theconveyor 5, while including straight sections, coils upward, that isperpendicularly with respect to the drawing plane 1, until it leaves thehousing 14 a of the dryer 14 again in the upper area.

The drying of the coating agent 12 in the dryer 14 occurs in air andunder mild conditions using low temperatures below 60° C., andpreferably a temperature of 45° C. A device 15 is associated with thedryer 14, which device dehumidifies the air used for the drying,preferably to a value of less than 2-3 g water per cubic meter air. Inthis manner, an optimal complete drying of the coating agent 12 on thehollow body 2 in relatively short time is guaranteed, for example, inapproximately 3 min.

The hollow body 2 with the completely dried gas barrier layer 3 leavesthe dryer 14 in the upper area and reaches a second coating device 16,in which the polyvinyl alcohol gas barrier layer 3 is covered by anadditional layer, which increases the mechanical resistance and theresistance against water. In the second coating device 16, the coatingbased on polyvinyl butyral or another alcohol soluble coating agent, forexample, is applied. The coating can also occur by blowing, where onemust ensure that the coatings overlap, so that the gas barrier layer 3is also covered in the transition to the uncoated collar 2 b. Polyvinylbutyral requires a nonaqueous solvent, where it is preferred to useethyl alcohol (ethanol).

After leaving the second coating device 16, which can be followed by adrip section (not shown here), the hollow bodies 2, which are nowprovided with the gas barrier layer 3 and a covering layer, are led intoa second dryer 17, whose design is substantially identical to the firstdryer 14. The dryer 17 also has a closed housing 17 a, into whoseinterior the conveyor 5 is screwed, this time from top to bottom. Thedrying of the second coating in the dryer 17 also occurs under mildconditions using warm air at approximately 45° C. with approximately 3 gwater per cubic meter, where it is possible to omit an airdehumidification device. Instead, a recovery device 18 for theevaporated solvent of the second coating is provided. After drying thesecond coating, the hollow body 2, with finished coating, is transferredthrough the conveyor 5 to a delivery conveyor 19 or directly to itsadditional processing machine, for example, a stretch blow machine.

As a variant of the described and illustrated embodiment examples, it ispossible to carry out the degreasing using other degreasing agentsand/or other known installations. Instead of the flaming device, one canalso use, for the purpose of subjecting a coating to a preliminarytreatment, corona, plasma or HF treatment devices which in themselvesare known, or similar devices. The discharging can also be carried outby other known means. The preliminary treatment section proposedaccording to the invention can also be used in coating devices for otherplastics and/or with other coating agents. Besides applying by blowing,one can also apply the coatings by spraying, immersion or other knowncoating procedures. The drying can also be carried out by other knowndrying procedures such as, for example, infrared radiation or microwaveradiation, in particular if the sensitivity of the coating agent isrelatively low. The invention can also be used for coating internalsurface, even when working with only one coating agent or when only onelayer is applied.

1. Method for manufacturing hollow bodies with a gas barrier coatingwith a coating agent having a polyvinyl alcohol base, where a hollowbody of surfaces to be treated is subjected to a preliminary treatmentto increase surface energy, coated and then dried, comprising amulti-step preliminary treatment, where the hollow body iselectrostatically discharged after the increase in the surface energy.2. Method according to claim 1, where the surface energy is increased toa value above 60 mN/m.
 3. Method according to claim 1 or 2, wherein thesurface energy is increased by flaming.
 4. Method according to claim 1,and an additional preliminary treatment with a fat dissolving agent,which preliminary treatment is carried out before the treatment toincrease the surface energy.
 5. Method according to claim 1, wherein thecoating is carried out by blowing the coating agent against the surfaceto be treated.
 6. Method according to, claim 1, wherein the drying iscarried out with warm, dehumidified air at a temperature of less thanapproximately 60° C. and with a water content of less than approximately3 g/m³.
 7. Device for the manufacture of hollow bodies (2) with a gasbarrier coating, in particular containers made of PET, in particularwith a polyvinyl alcohol-based coating, comprising a device (8) toincrease the surface energy of the surface to be coated, a coatingdevice (10), a dryer (14), and a multi-step preliminary treatmentsection (6) having a device (9) for electrostatically discharging thesurface (2 c) to be treated, which device is arranged after the device(8) to increase the surface energy.
 8. Device according to claim 7,wherein the device (9) for electrostatically discharging is an airshower with ionized air.
 9. Device according to claim 7 wherein thepreliminary treatment section (6) contains a device (7) to degrease thesurface (2 c) to be coated, which device is arranged before the device(8) to increase the surface energy.
 10. Device according to claim 7, anda film formation section (13), arranged between the coating device (10)and the dryer (14).
 11. Device according to claim 10, wherein the dryeris a warm air dryer and contains a dehumidification device (15) for thedryer air.
 12. Device according to claim 7, and a second coating device(16) follows immediately after the dryer (14) for drying the gas barrierlayer, for applying an additional layer which covers the gas barrierlayer, and in that an additional dryer (17) for the second layerfollows.
 13. Method according to claim 2, where the surface is increasedto a value above 760 mN/m.
 14. Method according to claim 4, wherein thefat dissolving agent comprises ethyl alcohol.
 15. Method according toclaim 6, wherein the air temperature is less than approximately 45° C.